1. Field of the Invention
The present invention broadly relates to articles comprising a thermoformable composite comprising a core comprising a renewable polymer and a heat-resistant outer layer substantially surrounding the core and comprising a heat-resistant polymer. The present invention also relates to a method for coextruding the heat-resistant polymer outer layer and renewable polymer core to provide the thermoformable composite.
2. Related Art
Polylactic acid (PLA) is increasing in favor with consumers of plastic thermoformed articles as a renewable plastic which does not derive from fossil fuels and which is degradable in the environment. As with many thermoplastics, PLA has a decreasing mechanical strength with increasing temperature. At higher temperatures approaching about 140° F. (60° C.), an article formed from PLA may lose the ability to resist deformation by forces frequently found in transportation. At temperatures above about 140° F. (60° C.), PLA may lose its ability to resist deformation to forces of the order of magnitude of gravity and residual mold stress. Prolonged exposure of PLA articles to temperatures of about 140° F. (60° C.) or higher may cause these articles to deform substantially from their original shape under forces which may be present in storage conditions. Since temperatures of about 130° F. (54.4° C.) may be exceeded in railcars and trailers used for distribution, PLA articles may suffer from high damage losses during transport through and storage in hot areas such as tractor trailers crossing, for example, the sunny warmer portions of the United States during the summer.
Accordingly, it would be desirable to develop PLA-containing articles which have greater resistance to deformation at higher temperatures that may occur during, for example, storage and transportation in warmer, summer time periods.